1. Field of the Invention
The present invention relates to a thermal printing head used mainly in a thermal print recorder.
2. Description of the Prior Art
FIG. 4 shows a peripheral structure of a heat generating unit of a conventional thermal printing head. With a thin film system, a glaze layer 102 is printed on an insulating substrate 101 and annealed. Thereafter, a heat generating layer 103 is formed by sputtering. On the heat generating layer 103, layers of a common electrode 104 and a discrete electrode 105 are formed by a vapor deposition method or a sputtering method. They are then etched into a desired pattern. Thereafter, the heat generating layer 103 is etched into a desired pattern and isolated to form a heating element array. Further, a protecting film 106 is formed by a sputtering method on the same. Eventually, heat treatment is performed at 500.degree. to 600.degree. C. to stabilize the heat generating layer 103 and ensure an ohmic contact exists between the heating element array and the common and discrete electrodes 104, 105.
With a thick film system, the procedure is basically the same as that of the thin film system except that a printing-annealing method is substituted for the vapor deposition method or the sputtering method. In this case, however, the a minimum annealing temperature of 800.degree. to 900.degree. C. is required. Usually, a ceramic substrate such as alumina is used for the insulating substrate 101; a glass of a high melting point for the glaze layer 102; Ta-SiO.sub.2, RuO.sub.2 or the like for the heat generating layer 103; Al, Au or the like for the common and discrete electrodes 104, 105; and SiAlON, SiON, amorphous glass or the like for the protecting film 106.
Conventionally, since heating at at least 500.degree. to 600.degree. C. must be required in the aforementioned manufacturing process to form a heat generating unit of a thermal printing head, an expensive ceramic substrate must be used for an insulating substrate to withstand the heat. However, the ceramic substrate has poor processability. Accordingly, the formation of circuit patterns for conducting electricity to the heat generating element is limited to one major surface of the substrate. Thus, the circuit patterns are multilayered and complicated. For example, Examined Japanese Patent Publication No. 52073/1984 discloses a thermal printing head in which a thick film circuit and a thin film circuit are put one over another on the surface of a ceramic substrate. Further, Examined Japanese Patent Publication No. 2627/1984 discloses a thermal printing head in which a multilayered circuit is formed on the surface of a substrate.
Further, the poor processability of the ceramic substrate makes it difficult to integrate a drive control IC for driving a heat generating element and other electric parts into unity on a substrate having the heat generating element.
FIGS. 5 to 7 show an overall structure of the conventional thermal head. As shown in FIG. 5, the insulating substrate 101 formed with a heating element array 103a and a hard printed wiring board 108 (usually, a glass fiber substrate is used, and it is referred to as "PWB" hereinafter) to which a driver 107 for drive-controlling the heating element array 103a is affixed by die bonding are affixed to a heat radiating board 109. Thereafter wires are bonded to it so as to electrically connect the insulating substrate 101 and the PWB 108. Referring to FIG. 6, after the heating element array 103a is formed, the insulating substrate 101 is integrated with the driver 107 by a wire bonding method. Further, a face down bonding method or the like is pressed against a FPC 111 (flexible printed circuit) which is bonded to a reinforcing board 110, upon the heat radiating board 109 through rubber 112 so as to come into contact with each other. Thus, the insulating substrate 101 and the FPC 111 are electrically connected. Referring to FIG. 7, the insulating substrate 101 formed with the heating element array 103a similar to that of FIG. 6 and integrated with the driver 107 and the FPC 111 which is bonded to the reinforcing board 110, are thermally pressed to come in contact with each other by solder. Thus, they are electrically connected. In the structure of FIG. 5, with regard to those which have been evaluated as nonconforming articles as a result of an electric test, after the insulating substrate 101 and the PWB 108 are affixed to the heat radiating board 109 and wires are bonded thereto, the insulating substrate 101, the PWB 108, the driver 107 and the heat radiating board 109 are bonded all together. Hence, it is impossible to exchange some part alone and restore the integral. They must be thrown away. Thus, there is a lot of loss in cost. In the structures in FIGS. 6 and 7, the electric test can be performed at the step where the driver 107 has been mounted on the insulating substrate 101. Thus, even if it is evaluated as a nonconforming article, only the insulating substrate 101 integrated with the driver 107 may be thrown away. However, in the structure of FIG. 6, the FPC 111 and the insulating substrate 101 are pressed to come in contact with each. Therefore it is necessary to provide a structure to hold the rubber 112. In the structure of FIG. 7, it is necessary to design a step of thermally pressing the FPC 111 and a terminal portion of the insulating substrate 101 to come in contact with each other by solder. This causes increased cost.
With regard to a process of manufacturing the heat generating element, it includes many steps under the present conditions, and it is desirable to decrease the process steps.